Shedding light on sunscreen biosynthesis in zebrafish

J ust as many people apply sunscreen before they go to the beach, bacteria, fungi and algae that spend much of their time in the sun also need to protect themselves from solar radiation. These organisms make small molecules that act as sunscreens by absorbing harmful ultraviolet (UV) radiation. Small-molecule sunscreens—which include compounds called the mycosporine-like amino acids (MAAs) and a related compound called gadusol—are also found in corals, marine invertebrates and fish. Sunscreen compounds like gadusol and the MAAs are important for the development, lifestyle and health of marine-dwelling organisms. For example, gadusol is found in high concentrations in the roes of fish, such as cod (Plack et al., 1981), as well as in sea urchin eggs (Chioccara et al., 1986), and it has been suggested that gadusol helps with embryonic development in these organisms. Sunscreen compounds also perform an array of roles in other organisms, including enhancing the UVlight vision of mantis shrimp (Bok et al., 2014). It was thought for many years that the ability to synthesize small-molecule sunscreens was limited to microbes, and that higher marine organisms obtained these compounds exclusively from their diet. Now, in eLife, Taifo Mahmud and co-workers at Oregon State University—including Andrew Osborn, Khaled Almabruk and Garrett Holzwarth as joint first authors—show that zebrafish can synthesize gadusol (Osborn et al., 2015). They also present evidence that the pathway used by zebrafish to make gadusol is distinct from the pathway used by microorganisms to synthesize MAAs. Moreover, they show that the gadusol biosynthetic genes are also found in the genomes of birds, reptiles and other organisms. In microorganisms that biosynthesize MAAs, the biosynthetic pathway involves three core enzymes and uses a compound called sedoheptulose-7phosphate as the starting material (Balskus and Walsh, 2010). The first step is performed by a dehydroquinate synthase-like enzyme. This enzyme is a member of a family of enzymes called the sugar phosphate cyclases, which catalyze the conversion of sugar molecules to products that contain a structure called a cyclohexane ring (Asamizu et al., 2012). Dehydroquinate synthase has a central role in the biosynthesis of aromatic amino acids, and is found in many branches of the tree of life—such as bacteria, archaea, fungi, plants and algae—but not in vertebrates. Given the lack of sugar phosphate cyclases in vertebrates, the Oregon State team was surprised to find a gene that encoded a related enzyme in the genomes of fish, including zebrafish (Danio rerio), a well-known model organism. This gene was similar to the EEVS (short for 2-epi5-epi-valiolone synthase) genes involved in the Copyright Brotherton and Balskus. This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.